Electric impulse signaling system



Jan. 9, 1945.

B. M. HADFIELDS ELECTRIC IMPULSE SIGNALING SYSTEM Filed Dec. 30, 1942 5 Sheets-Sheet 1 INVE'NTOR BERTRAM MORTON HADFIELD ATTO RNEY Jan. 9, 1945. B. M.-HADFIELD ELECTRIC IMPULSE SIGNALING SYSTEM Filed Dec. 30, 1942 3 Sheets-Sheet 2 w v 4mm M Tm X m Jan. 9, 1945.

B. M. HADFIELD ELECTRIC IMPULSE SIGNALING SYSTEM 5 Sheets-Sheet 5 Filed Dec. 30, 1942 D L E Y mm m m m V mm Z A Du 0 M f M A R T R 0 Y .B

53.28 E min mmkzaou mjmnon F0523 :3 20mm UNITED 'STAT -loop signaling trains of impulses to energize Patented Jan. 9, 1945 ES" PATENT OFFICE I l ELEcTRIonviruLsE SIGNALING SYSTEM Bertram Morton Hadfield, Harrow Weald, England,

assignor to Automatic Electric Laboratories Inc., a corporation of Delaware Application December 30,1942, Serial No. 470,659

In Great Britain February .28, 1942 I 9 Claims. "('01. 179-16) The present invention has reference to the immovement of the signaling methods and circuits pulses of known type. The invention relates more specifically to the present system known as such a manner as topermit the use of the normal supervisory arrangements. The terms near and distant as used herein refer respectively to the positions of the controllingcontact and the controlled relay orother device with respect to the transmission medium, such as a cable pair; According to one feature of the invention, in a function may take approximatelyflthree or "four times as long to reach a substantially steady used in automatic telephone and thelike systems whereby selection of the desired apparatus is obtained by means of make and break imiii and one ofthe objects is the provision of signaling methods and circuitsin;

signaling system, impulses are transmittedto a distant controlled relay through which battery is fed to-a line or cable over a loop circuit by alternatelyclosing the circuit at the near end of theline or cable and inserting a counter! electromotive force in the circuit to cause the current through. the controlling relay to revert toits value prior to the closure of the contact andso cause the relay to restore.

According to another feature of 'theinvention in a signalingsystem impulses are transmitted to a distant controlled relay through which battery is fed to a line or cableover a loop circuit circuit at the near end by alternately closing the and inserting thereat a counter-electromotive force, the value of which is so related to the electromotive force of the battery at thedistant end as to produce the desired static line conditions.

. art. However -by allowing It will be appreciated by those skilled in the l art that the above features of the invention do not preclude the normal operation of the circuit as regards supervisory signals, since thesetake place as a result of, orduring,

The invention therefore only changes the circuit from normal when the contact is opened, and as this latter effect is the basis by which such systems provide the selective the distant apparatus, itwill be seen from the following that the invention enables transmission of. such impulses to be effected with less distortion than heretofore. i

When loop im ulsing methods are used on conditions are normal.

transmission l'nes whose reactance is appreciable, i t such as underground cable pairs, the arrival and decay current/time functions at. the distantend are greatly different.

the closure of. ,the near contact, and in this state the circuit For instance the decay as a counter-EM. so as to current to the non-operating condition. In this manner the decay current/time function can be made similarin type tothe'arrival function so thatfor a given lengthof line the speed of sigof the receivedimpulses are established.

consists of'making the of substantially twice arrival function, and

state, compared with the. waveform with time.

moreover has a different This isdueto the change in the condition of the cable from short circuit to open circuitat the near end. Henceaccurate reproduction of the impulses is extremely difficult, owing to the in: herent differences intheloperating waveforms and the mutual interference between them as the speed of signaling or thelength of line is increased} These effects are knownffor instance in submarine cable telegraphy, and in order to overcome them 1 resort is normally made to a signaling system whereby the battery is brought to thesame endjof the line as the contact, .softhat successive applications of battery and short circuit are: possible.

As central battery systems are practically essential inautomatic telephony. for transmission of backward signals his not readil practicable to have i the battery at the impulsing if control end for reasons well known the openingof .the contact to insert a counter-E. accordance with the invention, the same effect is obtainedin that the signals now consist of successive applications of. a short circuit and a battery,the diiference being that the. battery acts restore the line for this reason that i a specific embodiments will now be describedwhereby the invention may be better understood. Referring to the drawings, Fig; 1 illustrates ,the invention inits simplest form. Fig. 2 illustrates diagrammatically an application of the invention when an earthiconnectionyis present in the counter electromotive force. Fig. 8 shows the invention appliedat thenear end to a circuit of course well tothose skilled in the M. F. battery in i in which supervision takes place and the coun ter-E. M. F. equals the E. M. F. of the distant battery. Fig. 4 shows the invention applied at the near end to a circuit in which supervision takes place and the counter-E. M. F. equals twice the E. M. F. of the distant battery. Fig. 5 shows a circuit arrangement )fDI' relief relays efor the arrangement :shown in Big. 4. vFlig. i6 :shows .an auxiliary circuit for use with the arrangements shown in Figures 3 and 4. Fig. 7 shows a complete circuit embodying certain refinements not present in the arrangement shown in Figs. 3 and 4. Fig. 8 illustrates various circuit conditions under busy flash and manuahhold signaling. Fig. 9 illustrates a manual board for signaling back over a connection employing impulse repeatersaccording to the invention.

In the following descriptions it will'be assumed that the normal transmission bridges for iCQHfiI'lingithe steady state D. C. signals tothe given line are of the transformer type. Although the in- Nention can libe applied :to the 'more usual condenser ;transmission bridge system, .it new considered that the latter "has .so marry disadvantagcs due to transmission of longitudinal surges caused by signaling in the various portions of the complete speech path and due to the "very inductive nature of the circuit under impulsin-g conditions, thattits .usezwhere accurac of impuls'ing ,periormance is desired is iinadnnssible. I will also *be. assumed thatfthe line over which signaling is required consists in-general of two wires havingthe usual distributed constants and balanced with respect to earth, and that the most significant constants are ithe series resistance and shunt-capacity.

In these circumstances and as the battery at the distant .end although :ear'thed -is connected to the centre point of the terminal impedances, no dilficlflty :arises .in "the method of connection 0f the near contact .and its counter-E. M. battery provided 'neitherhave an earth connection. "I he contact may consist, for instance; cf a single 'changeover in which the'movingspringand make .contactare respectively joined to the ends of the .near end wires via "the transformer bridge and the usual supervisory relays, whilstthe breakcontact is joined to 'the pole ,of the counter-E, F. "battery of 'similarpolarity to that received from the distant endon the moving'sprin'g. The other ,pole of the counter E. battery is domed to the make contact, i. e. to the other wire. Such an arra'gement is illustrated iin Fig. 1 in which the transmission path over which signa ling-is desired is represented by the dotted pair :of lines joining the terminal transtormers lil andTl, the pad) being of a nature capa'blezo'f passing-direct currents and consisting for instance, LOf .:a cable pair. The transformers isolate this :path from "those preceding :and following :as :regards the steady state 1). signals but :permit through transmission of audioifrequency currents. The direction of transmission of 1orwa-rd signals such as trains of dialled impulses is from left to :night, :and relay IA may be regarded as responsive to such signals "by $118 passed through it from battery 4E1 :under the control of :a .previous contact. "The condensers Cl and C2 reduce the :impedance of the relay in and its counterpart .Z-A zat audio freguencies .so that the latter may be transmitted withoutsubstantialloss. L'IZhen n tact ML] of :relay LA is shown in .the zunonerated condition and it will be apparent :that the coun- {ten-E. M. battery .EC opposes .thefiowof current from the distant battery E2. If the E. MSFLs of EC and E2 are substantially equal then no line current flows, whilst if the former be twice the latter then a current due to the difference will flow from the former. When the relay IA is operated EC is removed from the line and the cur rent which flows is due solely to E2; and causes relay in to operate. Hence .when EC equals E2 single current operation of 2A is obtained and when EC is twice E2 double current operation is present so that relay 2A may then be of polarised type, but with either condition the send- =ing endof the lirre is always of minimum impedance ,thus producing operate and release Wavefronts 'of identical 'form and of minimum time duration. .Also it will be noticed that when the circuit is in use and relay IA is held operated, conditions :areithe same as for normal loop signaling methods so that normal supervisory arr-angements and circuits may be used.

As is well-known the best possible conditions for accurate "reproduction of the 'near end impulses bythe distant relay are given when the *received currents have equal "positive and negative values, e. when the cQunter-E. M. 'F. is twice that of the distantbattery. When the invention is applied iinvthis preferred manner, due allowance .must be "made in the near end supervisory relay circuit design for the reverse'current which fiows'when the contact is =un'operated, sinceun vdcr the existing signal 'system the "line current is reversed byareversal of the distant battery when the called subscriber answers in order to effect metering at the calling end. This' point will be dealt with in greater detail after the following description :of the methods "used to :apply the ,invention "in the case 'where "the counter-E. M. F. battery may have an earth connection. It "is obviously desirable that :this condition shall "o'b- .tain in practice, in order that .the potential of this battery shall be .stabilised with respect .to.

earth so that it .canbe 'used as a common supply fora number of lines.

LReterring .to Fig. '2, this figure shows a gen eralised circuit 'for the transmission path. Impedances Z1 and Z2 .represent the necessary terminal impedances such .as transformers and su pervisory relays, resistances ,-r and capacities Ccv .CI I represent an elementof the lines distributed constants, the voltages .of the near ends of the Jines with respectto earth.arep-andtheline currents which i-loware ii and i2, where i! is due .to 'e.| when :the contact [Al 'is ,closedand ,i2 flows when the contact is open. cl is .the E. M. E. of the distant battery and it is assumed that its positive ,pole is 'earthed. Let :it be assumed :that the counter-E. M. ibattery consists of two series aiding components ,zeZ :and c3 of total F. greater than el, having the Junction of c2 and c3 :earthed, :and :so disposed that :the positive pole of oz can :be joined to .the positive gnear .end wire and :the :negative pole of e3 to :the :negatilve :near iendwire via :two resistances 112 :and =13 respectively. Let :con-

be joined to the near end wires at the same points to which it is possible to join c2 and c3. Let the necessary terminal impedan'oes, relays and the like be disposed symmetrically with respect to earth and be included as portions of the cable pair. Let the effective constants'of the cable be represented symbolically by equal series line-resistance, equal capacities to earth, and line to line capacity. Then if the contact be operated without the connection of c2 and c3 "to the lines, when the line current attains its Butwhenthe contact opens e2 will now drive current into the upper loop via the earth, being aided therein by the charge Q steady state Ivalue the potential of the wire connected to the positive poleof thedistant battery falls uniformly from zero at thedistant end to at the contact with respect to earth. The potential oi the wire connected to the negative pole of the distant battery rises uniformly from el to adding either two small groups of cells each substantially equal to atthe contactlwith respectto earth. Hence both nearend wires are at the same potential of,

tials, and that ,whenlthe contact is opened no resultant earth current is initiated. Obviously the former requirement is met by a variety of valuesfor e2, as, T2 and,r3,since.wehave:

"on the upper wire with respect to earth, and e3 will perform .a like function for the flower loop but is opposed by the corresponding charge on the lowerwirewith respectto earth, In order that no earth current shall beinitlated it is necessary that the sending current/time functions of the two loops shall be identical and that the magnitudes of the currents shall be equal. 1 Now i the formerrequirement is met onthepart of the f mcable wires and, terminating impedances by as-- sumption, so thatitis onlynecessary to make r2 with respect to earth. It is necessaryto arrange the connection of c2 and e3to the wires so that J i no change is effected in these steady state potentroduction of the feed resistances. If these re-. sistances are small compared with the line resistance the difierence between the functionsis to each end of thelocal battery, or the use of a twin output mainsrectifier unit in place of the additional cells. In either case the internal impedances of. the additions will form a part of the feed resistancesand should be so calculated. ,In applyingthe invention to a given piece ofapparatus, such as for instance a relay set,

either of the above methods of signaling can be used at will if the feed resistances are connected to U link pointswhich can be strapped either to the local battery or the additional portions connected thereto. l

The above method of applying the counter E. M. F. via equal feed resistances does ofjcourse,

result in the decay current-time function at the distant end being slightly longer than the arrival current/time function, owing to the invery small, and of course becomes relatively unimportant as the length of line is increased, for a given'value of ieed resistance. As the effect on equal r3. The second requirement taking into above steady state equation. H

i The above are the requirements for the genseral case, and ensure'thatjno earth current flows so that crosstalk and other mutual interference between pairs remains at a minimum. In particular for single current signaling (i. e. line]; current changes from zero to maximum in one direction) it is obvious that. the sum of. e2 and e3must equal el, so that e2 becomes zero and e3 equal to e l. Thus it becomes possible to use thebattery whioh normally exists at the near end .for operation of the relay contact asthecounter- E. M. F. since as e2 is.zero this meansthat its positive poleis earthedpand also-provided the account the fact that "r2" must equal 7'3, gives I es-e2 =el, which will also be found to satisfy the the longer lines is the most important consideration (sincethe current time functions onvery short lines aremuchfaster, and small differences are therefore unimportant), then the use.

of feed resistances of say 200 to 500 ohms, is permissible. In additionjand more particularly in the case of double current signaling, a definite use can be made of such small difierences brou ght about by a limitedchange in the sending end resistance, inthat the operate transit time of the distant relay can be approximately compensated by intentionally causing the decay function to .be slightly longer. 'lhe reniaining embodiments of the invention will describe means whereby the normal supervisory relays and functions asso sciated with loop signaling may be incorporated ,Without detriment to the described methods of impulsing. It is usual to associate with the send?- ing contact four relays known asthe I, D, Band C relays. The first two are essentially operated by thesamelinecurrents as operate the distant impulsing relay, except that the D relay onlyo perates when the distant battery is reversed as regards the line potentials. It follows that these relays must be considered as part of theterminating impedances associated with the lines. ,On the other hand theB and C relays although operated by the local contact, perform this function from the local battery and therefore do not affect the impulsing arrangements, except in so far as to practical differences in potentials of the two 1 "batteries are-suchthat the ensuing residual line .Clll'lllt does not materially eiiect theto eration of the distant relay, andin particular does not operate thelatter when the contact is open.

As regards double ,current signaling where the line current'changes between equalpositive and "negative values,the sum of .e2 and e3 must be twice el, that e2 becomes one halfel and c3 becomes one and a half times all ,These values can be obtained with :economy'in practice by. 1 using the localbatterylsupply as a part ofe3 and circuit of the invention will first be described with reference to Figs. 3 and 4 which illustrate and require a second. contact operating in sympathy with the line contact in the usual arrangement; i

It will be shown that the need. for this second contact disappears as does the need .forl the C relay, since the mainfunctionof the latter is to short the I and D relays after the .first impulse and during the remainder of the impulse train,

and it is proposed to, leave these relays in. circuits The connection of the Land D relays in the the near end termination only for-single double, current working respectively.

Referring to Fig. 3 thesupervisory relay l is non-polarised while the supervisory relay D may be polarised or non-polarised. When D is non- .polarised then a rectifier MRI *mustbe connected so :as "to Joy-pass the :normal .line current and ?b e operative only with reversed currentfromthe distant end when the called subscriber answers. In these circumstances a resistance r4 shunted by a rectifier MR2 must be inserted as shown so as to keep the near end line impedances balanced for impulsing. 'When D is'polarised neither rectifiers 'nor additional resistance are required but 'a retaming spring or bias must be used to position the armature.

The relays I and D are inserted in the respective line connections between the contacts [Al and the ends of the windings of transformer Tl, whichare connected 'to C2.

It has been pointed out in the description of the arrangements used when the counter-E. M.

F. has an earth connection, that it isessential to keep the lines symmetrical with respect to earth. It follows therefore that impedances can be introduced into the line side of near contact, provided each line has the same added impedance and their values are 'sulliciently low in comparison with the line when the latter becomes long,

and that they are preferably resistive in nature. Furthermore both I and D relays must essentially have the same operating sensitivity as the distant relay, and all three must be sufficiently sensitive in-terms of the energy required to operate them to allow of the use of low resistance windings so that their eilect onthe line characteristics shall be a minimuin'and their voltage drop shall not require the use of large battery voltages 'on long lines. Hence it is proposed to use two relays'of exactly the same type as the distantrelay for the I and D relays and of resistances of the order formerly quoted as being suitable for "feed'resistance, i. -e., 200 to '500ohms. Each relay is shunted by a series condenser CI and CD respectively and resistanc'es'RI and RD respectively of values such that the nett impedance of each rer lay is resistive in the well-known manner. The

determination of these values is arrived at experi-,

mentally by applying an in'terruptedD. C. voltage to the relays via aresistance and adjusting the shunt condenser/resistance until waveform across therelay or the current waveform applied thereto becomes substantially rectilinear. The types of relays to be used depend on the type of impulse signals and will be discussed later.

It will be seen that with the above method there is no need for the usual practice or shortcircuiting the I and D relay, so that al1 impulsesrece'ived at the distant end are similar as far 'asthe line permits and also that the C relay can be dispensed with. The normal B relay must however be provided since one of its func-- tions is to busy the apparatus as soon as it is seized and irrespective of Whether the distant relay is being operated. The B relay can take 'tlieplace of the two feed resistances used to enable the counter-'E. M. Rbattery to be applied to the lines as before described, provided it has equal windings and its impedance is rendered resistive as described for 'the I and D relays. If, as is usual, the B relay has a long release lag, obtained by a heel slug this provision may not be fully necessary, as the inductive component of such relays is very small owing to the 'shortcircuited turn. The efiect of the above described method for rendering the I, D and B relays substantially resistive so that they act as feed resistances and do not materially affect the signaling current/time functions as the distant the voltage shunting the relays individually withta circuit ielement possessing a .high power law relationship between the current passed by the .element and the voltage applied thereto. In a known form of this type 'of element the current is proportional to the fourth or fifth power of the voltage, so that the initial current surges required at the sending .end ,of'the line whose value greatly exceeds the normal line current willbe by-passed from the relays. In this manner the desired effect of a low sending end impedance can be maintained.

The arrangement of Fig. 3 therefore includes an incomingline relay FA feeding battery from source E1 back to a position where an impulsing contact is located, and transformer Tl, condenser Cl, all as in Fig. '1.

The counter-E. M. F. is derived from the battery El (having an earthed positive polar-and two smaller'batteriesEz connected in series with El, one on each side.

U "link arrangements ULI, UL2, UL3,UL4 are provided to permit of single or double current working at will (compare the connections of ULI, ULZ, UL3, UL4 in Figs. 3 and 4). The double-wound slow release relay B which is rendered -resistive-by means 0f the resistances RB and condensers CB shunting its windings is connected between the counter-E. M. F. and the contacts zlAl the :circuits from the other side'of the contacts LA! to line have already been :described.

Fig. 4 shows an arrangement for double current supervision in which the I relay whether polarised or not has .a rectifier .MR3 .for .the .purpose of' by-passing the reverse line current when the contact .is .open. This relay therefore energizes when line relay lA operates, remains operated during .impulsing due .to the short-circuit effect of MR31on its windingand releases when the line current is reversed on the called-subscriber replying :In Fig. .5 relief relay IIand DD .for relays I and D .respectively have their circuits Iso arranged .that .a rectifier MR4 connects their .free terminals, together whereby relay II is .-.energized,.permanently throughouta .call i. ,e. .whenever either relay I or .relay'D is operated. The release of relay 1 when the called subscriber answers is a new facility in the art and although it may be used assuch, the relief relay II which is permanently operated can take over its normal functions.

.As regards the D relay circuit when using double current 'impulsing, it will be appreciated that it is required to distinguish between the reverse current normally flowing (i. e. when -the near contact is open) and the reverse current which flows when the distant battery is reversed.

'In the first case the D'relay is not required to operate and in the second it must operate. "The change of state of the near contactmay be "used to perform this distinction. For instance, if the D relay be of the non-polarised type and the changeover contact operating in sympathy with 'the near contact be available, then the latter can'be arranged so as to shuntrectifiersfin turn across" the relay with its operation, the vre'ctifiers being so connected that the impulsing line current always passes 'therethrou'gh. despite the reversals. When .impulsing has finished and the distant battery is reversed this cannot affect the near contact and therefore the 'D relay operates. For similar reasons as applied to the I relay ;it will in general be preferred to use .a polarized rel-ay-for the Drrelay and this enables the extra I near contact to be eliminated, since by connect ing'the relayso as not to be operatedlbythe normal line current '(i. c. When the near. contact I a is closed) the rectifier performing .thisfunction formerly can be removed together with the contact. In particular it is arranged as shown in Fig. 4 that the moving spring of the near contact is connected to the line in. which the D relay occurs, and the make contact to the other line (i. e. the junction of the l. and B relays). Then the break contact "is connected tothe other side of the D relay via a rectifier MR5 of such polarity that when the nearcontact is unoperated the normal reverse current passes therethrough; At the end of impulsing when the near contact retotal primary and secondary ampere-turns. The

rectifiers MR6, MR1, MR8, MR9 are connected in I circuit by cojn'tacts DDI .to DD4, iand it is arranged in Fig. 6 that relay DD is operated by contact I2 and released by contact D2 thus ensur.

me that they are only in circuit for the period mains. operated then the D relay is ready for; .1

operation by the subsequent distant battery reversal. I I

It should be explained that the arrow shown in the relay D indicates the direction of current between seizure and answer. In these circumstances therectifiers are not Ecalledon toi-pass relatively heavy currents for. long duration, and the only requirements are that they shall Withstand the maximum steady backward, volta e due to the linecurrents passing through the windings (in general about lvolt) and havethe lowest possible .fo-rward resistance consistent Withia back- Wardresistance. giving the permissible additional transmission loss to supervisorytones. It has to cause the operation of therelay. The armatures of I and D when. polarised must be posi- I tioned by retaining springs or biases. u u

It will be noted that normally current flows through the Brelay in Fig. 4, that is when contact lAl is back, as the counter-E. M. F. exceeds the battery E. M. F. at the distant end. This current is however very much smaller than the currentwhich flows through its windings when the contact IA! is operated, This involves; a certain amount of marginal design for relay B but on the other hand it simplifies theproblem of holding during impulsing. .Inthe worst case of ailine having aero iresista nce it is possible to arrange that the change in B relay current is not less than 4 to 1 which enables a practical design to be achieved.

Use is made of transformerytype transmission bridges .for coupling togetherthetpaths for audio frequency signals, in order that .no. longitudinal impulsingksurges may interfere with theper- .formance of any one path. .However. it is still possible for transverse surges to provide'interuse of such rectifiers also prevents the transmission of all surges via thespeech path so that from 7 been foundthat the types used as acoustic shock absorbers in suitable. I

. Apart from thequestion of being able towork each signaling path under ideal conditions, the

this point of view, impulsing overamplified junctions inwhich the D. C. {path is taken via the quad. circuit canjbe carried out, without the probability of over-loading. the amplifiers.

H The elementaryfcircuits sofardescribed have certain disadvantages inpracticeand Fig. '7 shows an alternative circuit designed to remove or reduce these. i i In order Ito economise inlthe. description a specific circuitlayout for theO/G and 1/0 relay sets willibe described, using pertain preferred types of backward signal, but it should be understood thatother forms of backward signal. can beused and may have. advantages in certain cir- 'Jcumstances. The circuit will be illustra-tedby diagrams showing the line currents for the types I of backward signal to be used, and provision has ferenceof sufficient magnitude to alter radically the impulsing performance of one. path.- Such surges can be substantially and conveniently suppressed by the appropriate connection of metal rectifiers across the windings of the transformer during impulsing. It can also be arranged thatthe flow of line current when the l I circuit is in use and priorto theanswer signal,

u (a) Repetition of busyjfiash over. ordinary juncis in such a direction as to produce a voltage on the rectifiersin th backward direction, so

that the additional transmission loss during this period is negligible. llnaddition it can bewarranged that the rectifiers are disconnected. upon receiptofthe answer-signal, and that they are not connected until the circuit is seized, so that their required performance. is dictated solely by their ability tosuppress surges. I

As the surges will be of both positive and negative' polarity, it follows that :the rectifiers must be in pairs and connected back-to-back. IfbOth rectifiers are connected to one winding then one of them is bound to be conducting, so.that the transmission loss will be large. Howeverby connecting the pairs one "to each side of similar windings, arranging thatthe connection of the windbeen made for the following. in addition to the normal ,signalsassociated with seizure, dialling and answer as before described: I

tions, i.e., those between automatic exchangeshaving manual boards. (b)

junctions,;i. e., those between a. mainexchange havinga manualboard,and, an

exchange not having a manual board.

Re-ring facilities subsequent to manual held can ialsobe provided with little extra difficulty, I I

but will not be specifically described.

Fig, 'lshows these circuits in detail, and as it": will be seen that there is considerable similarity betweenithem, the same numerals and letters havebeen used in both for items which as far as possible, perform the samefunctions. The double current method of impulse signaling has been chosen over the cable pair, as this method is -fundamentally sounder, but the circuit operation r can just as easily work for single current signaling when the appropriate changes are .made in the counter-E. M. F; at the contact and in the ,l ype of. I/O relay as previously described. 1 All ings is straight through". (i .e., the alternating 1 voltages on similarwindings are in phase), and. ensuring that the line currents under seized conditions prior to the answer signal cannot pass through the rectifiers, as shown in Figs.,3 and 4, I

. all the desired. conditions can be met. In addition this-method of connection of the transcontacts are shown in the unoperated condition.

The outgoing relay setOGRis shown within the dotted lines onthe left-hand side ofiFig. 7.

Theconnections to the incoming pair are made atpoints I and 2, and it isassumed that this.

pair is looped either by acontact (as; from a subscriberfs instrument) orby asimilar contact I formers ensures that the normalline currents produce only a flux duetothe difference of the j telephone I receivers are eminently Manual hold and operator. flash ,on satellite I andapparatus as is shown connected between points 5, and '6' of the 1/0 set (via'a group selector). The speech transmission bridge consisting of transformer TI, condensers CI, C2, metal recti-' auxiliary circuit while resistances whose Value can be selected with sole regard to impulsing conditions are substituted therefor in the counter- E. M. F. circuit. I

Relay A which responds to a loop on the incomin'g pair and to dialling impulses,is of well known small time constant construction and has one changeover contact AI". obtained from the battery EI is fed via a barretter BRI, in order that it maybe of satisfactory value for a subscribers transmitter, should this set be seized by a subscriber. The contacts D2 and D3 relay the answer signal by reversing the current in the loop, in the normal manner; such operation only being effective, of course, when the loop includes apparatus responsive to such reversal (for instance it isnot usually effective when the loop includes a subscribers instrument). The contact MI and resistance R! are only inserted in a relay set terminating an ordinary junction and are for repeatingjbusy flash as described later. Comparing the" outgoing side with Figs. 3 and 4', it will'be seen that several alterations have been made in the relays, but that the position of the contact AAI', the feed resistances RI and R2, and the batteries" E2 and El are substantially the same. The contact AAI is, of course, located on the relief impulsing relay AA, the feed resistances are equal (some 400 ohms each being, found satisfactory), and the batteries E2 are of value su-bstantially one half El and may be small mains rectifier units as described before; Of the contacts D4, D5 and*'B3', B4 the= first-two relieve the batteriesof the local current when AAI is operated andspeech is in progress and also'permit thecall beingunder the control of thecalling subscriber, whilst 33' and B4 permit manual hold conditions to be established by' a backward signal. I

Relay ID'which takes the place ofboth- I and D relays in the line, has a balanced winding in each line to mitigate fortuitous earth current interference. Condenser/resistance circuits C3, R3 and C4, R4: are provided as compensation as described. before so that each winding of ID. acts as a series line resistance, and is shunted by rectifiers MR5 and MR6 so that it is only operative to currents from: the distant battery (when AAI closes). Additional rectifiers MEL-MR8 are generally advisable when the reverse current is large in order to prevent incipient operation of ID owing to the voltage drop across'the shunt rectifi'ers,

but can be arranged so that with MRB and MR6 they form single items. Relay IDis of normal telephone type and its sensitivity primarily defunctions of the I and D relays, by operating when AAI operates (if-the loop and battery'conditions -fromthe distant end} are correct) and releasing The current in this 'loop ajeease r when the answer signal of. reversed line current in the winding is meant toindicate that'the windings are crossed as regards normal loop currents, i. 'e. one line winding is reversed so that the flux produced by both windings when theline currents ar only of theloop type, is zero; Hence this relay does'not need compensating and only acts as a resistance on impulsing. It will, however, respond to a difference in line currents, which may be brought about by unbalanceof the circuit as regards battery voltages or other means; the formerbeing the method whereby backward signals from the distant end are detected. v i

As regards the local relay circuit shown detached, this willbe described by taking the operation sequence to a successful call, 1. e.- in" the absence of any backward signal other than the answer signal. The operation on busy flash and manual hold necessitates considerationofv the the A relay circuit ona satellite junction only and need not now be considered; BI reinforces the earth on the private via DE and maintains. it duringimpulsing; the operation of B3 and BI is ineffective at this stage. I2 preparesa circuit for operating D, I4 does the same for C, and I3 operates DD, thus connecting up the transformer rectifiers MRI, MR2,MR3 and MR4" via DD I to DD4 and preparing for positive battery metering via DD5.: Dialling may then: proceed, and on the first break o-fAt, relay Cis; energised and holds in for the duration of. the. train by means of its normal slow release feature; relay'B of'course also remains operated duri'ng impulsing for the same reason. 'Wh'en the called subscribe-r answers, ID releases and operates D since: C2

will now be normal. DI releases DD'and retains D Via I3, and during the release lag of DDthe positive battery E3tisapplied via 'R8 t'o the private to give the meter signal; MRI I retains. the earth on the private for the transit time of DD and, D.

Release of the connection by the: called subscriber only re-operates the in the 12/0 set (by restoration of the normalibattery at the distant. end-this efiect being: a repetition of the original restoration in the: finalselecton via the I/C. set) and since-botlnl and 'D in-bothsets are operated; has: no effect. Release bythe calling subscriber however, releases A; and :AA, operates 0,, releases B and then. I and then D; andv hence the WhOIB COI'EIIBQtiOIIL The private-is guarded for -a time equal to the sum of the releaseilags oiI-B,

that of D (thezlast relay; to be-releaeed; apart from v 'C) then adequate guard. is obtained; There is also adequate: guard tm ensure the release of. the

- incoming. relay setlCR andagroup. selectolt asrwill be described later. A51 regards the: t ming; requirements. of the. four release lag. relays therera'rerno: inten-locking 1T8:-

quirements, and thoseof B; GaindxDD' cambe ture to normal.

normal; the release lag'of -I is only required when Mis to be operated (see later) and must exceed the operate lag of the .latter, so that some 50 milliseconds is adequate. l

It might be. thought that there isa possibility that ID will impulse, but this is not so in practice for two reasons. adjustment of the relay is prejudicial to impulsing, and the other is that the operative current pulses when the AAI contact closes exceed the steady state valueon account of the discharge of the line capacity. In any case the C2 contact would prevent such. operation from becoming effective to nergise relay. D, although the inclusion of this contact is primarily made for the purpose.

circuit of the latter to compensate for the; combined transittimes of the A and .AA relays. This is the purpose of rectifier MR9 and? variable resistance R5,since as the former isfonly operative to theback E. of, AA (when AI releasesl then by adjustment of1R5 the release lag One is that,the required of a preventingthe calling subscriber fromme done on an effective loop/disconnection basis if considerable distortion is not to ensue. This A relay'will be of conventional telephone type,

bridged across the line, and possessing considerwill present a high impedance to this back E. M. F.

which as is well known, is of such polarity that it tends to pass current into the battery.

As regards the I relay, it will be noticed that this has no intentional long release lag andis directly under the control of the Acontact forree lease purposes. Thisis done in order to ensure the earliest possible release of ICE and its group selector. The group selector asseciated with ICE is arranged to be held forward by the earth on the private; for this purpose the continuity of the private wire through the group selector via the can be altered. to eirect such compensation. Rec- I tifier MRIU is necessary in order to protect AA 2 from the combinedbacklil. M. F.s of theother relays operated from the Al make contact, which would otherwise produce impulse distortion.

The incoming relay set ICR (shown in dotted lines on right-hand side of Fig. 7) is to a large extent precisely similar in layout and operation to the O/G. set, and so no detailed recapitulation will be given. Thedifferences will now be. described. i l

The A relay willnow be of the polarized type as double current working is being used, and it becomes necessary to consider whether there is the possibility of momentarily having no current in the windings, thus tending togive an indeterminate position to the armature. This condition is possible if the AA contactinthe O/Gsetis released beforeeither the contacts B3, B4 or D4, D5 have restored. This always obtains when the calling subscriber clears, except when clearing against a busy connection (since D will not U have been operated), and fora time dependent on the release lag of. B. It is the purpose of the third (middle) winding on A toensure that there is always a flux. due to the current from El. via R9 and DD6,-which tendsto restore the arma- This fluxisa small percentage of the normal flux and doesnot therefore inanormal, makeH relay contact, is removed by simply disconnecting thistcontact. The group selector is otherwise of normal type. Hence in DDS. Now in a normalsuccessful connection the I the earth on the private should relay I impulse 1 during dialling, and will ordinarily have been released by the answer. signal. Thus. the release of the group selectorwby the calling subscriber is fast, 's'incefit. is only dependent on the sum. of the release lags of the A andAA relays in the preceding 0 G set 0GB, plus the release lag of the presentJA relay,. and I relay. If the called subclearing from an inefiective call, since D will not scriber lreleasesfirst with consequent re-operation of ID, the release still follows the sameproceduresince D is held by its own contact Dir via The longest release lag is obtained when have been operated and the earth on the private will be maintained for the additional tihie of the release lag of DD. Under these conditions,

however, and since no answer signal has been i I. received; it is only necessary that the release lag many succeeding O/G set, in orderthat the group selector shall clear first. Thiscoridition of DD shall be less than the sum of 3,1 and C is obviously easy of attainment. v

terially affect the normal contactpress'ures, and

furthermore is only present when DD is unoperated. As the only time DD is operated is during impulsing and prior to the answer signal, itfollows that the impulsing performance of A is not.

affected by this feature. i l

The alternative connections of the MI contact and its associated circuits will be described later when thelbackward signalsare taken. 1 l

The rectifiers MR1 arrdMRB which were placed inseries with the ID relay in theO/G set, are now not necessary since there can be no reverse current as'we areusingth'e same batteryEl =for counter-E; M. i F. as is used for operating. the

. group selector or next O/G relay set. A A similar assembly canbe used however, because another pair .of rectifiers are neededas shown atMRll and MRIZ. Thesejlatterare necessary because itlxwill be needed. to. impulse the A. relayin .the

Since thelast discussed conditionzdoes give a longer release period for the group selectori than normal without ill-effects, it may be queried,

as to why the I relay cannot be given the same release lag and thus dispense with that"on DD and alsolthe DD5 contact. There are two reasons why thislis undesirable. The first is that should the calling subscriber re-call when ID will have of ID, when it will lock via its own contact DI and I3 and hence give the standard answer signalwith metering. Thisoould, of course, be

obviated by a small operate lag on D, but in View of the second reason is hardly worth considering; it being assumed, of course, that the chance of-a re-call during the normal release lag of I is negligible. Thesecond reason is that under manual hold conditions and whenthe subscriber clears, the present D relay must restore to normal beiQre.Lt1ie ,:B4 n s i the ,Q/ set ps beheld when the subscriber: hangs up).

otherwise the correct backward holding; signal will notice re'ceivediby the latlten As; the D- re'-- lease is: dependent" on. 1,. it follows that the latter cannot: have a. release lag. comparable. with B3.

whichv it would; have to haverif itlis. to hold over impulsi'ngz aeoascn laysg. is denoted by R and the: resistanceot one:

feed: resistance. by r; Thelcurrentfigures quoteda A spark. quench of some: form will be needed E. M. F. across. I on release may reverse in sign,

owing to the: oscillatory nature of the; circuit. As this relay is effectively in parallel with AA- such an. E. M. F. would cause MRl-El to become conducting, with consequent. detriment. to impulsing. Hence it is preferred to use arectifier. MRIS and series resistancev RID in shunt with I,. and operative only to. the back E. M. F. circuitis non-oscillatory and hasthe further ad-' vantage of permitting the adjustment of the re.- lease' lag of I to suit the requirement that DD shall hold over impulsing.

It'will be noticed that the D4. and D contacts. are not here supplemented by B contacts '(or contacts performing a like function) as in the case of the OGR set. This is because in the case of: the OGR- set it is desired to maintain the reversed battery answer signal, where: the outgoing set is.

located at a satellite exchange having. discriminating selectors, so that the latter may function correctly under manual hold conditions: (i; e. so. that the I relay'in the-discriminating selector may This. means that the D and .I. relays in the OGR set must beheld (which is done by Ml, but-see later), so that contacts B3 and Bdlthen become necessary in order that M may be operated when the subscribed clears. This is not necessary in the casev of the set ICE and in fact D must'be allowed to restore in order to transmit the proper hold signal backwards;

' line with negative to the other, respectively (the polarities are with reference to earth, remembering that the normal battery supply has its; positive pole earthed) Fig. 8 shows aschematic diagram of the essential line and termination. conditions, under the two backward signals to be discussed. It is necessary'to-consider the effects of such signals on both M and ID relays, and the latterhas been shown in abbreviated form as regards. the rectifiers; also the effects due to the line contact AA! being open or closed and the action with double counter-E. M. F. (i. e. double current: impulse working) and single counter-E, M. F. ('i. e. single current impulse working) must be considered. Again for the sake of' brevity although the bat teries which change the line conditions and bring about these signals have been lettered El and E3 to conform with Fig; '7; it has been assumed that both have avoltage-of and the line currents have been quoted with the upper figure on this basis. Also the total" ohmic resistance: of a line audits part of the terminationsincludihg the-res inside: the? relay M are. those which will produce: operation of this relay, 1;; e.-. the difference. betweenthei respective line currents since M is connected with crossed? windings; whilst their val-- uesare: assigned; plus or: minus: signs. which will;

- be. useful: when considering} the possibilities; oiv

selective backward. signaling. The counter.- E. M. Flbatteriesandieed resistances at the AAA. contacthave: not been drawn,, as they are precisely' similar to; those: previously described.

Now the: present busy flash: signal. is: obtained frontaiselector the. desired signaling path (ire. an intermediate group selectorl or aafinal selec tor), andiis: of; the: same? form as; that;proposed'.. Even although. present .policy' is to; dispense with. backward; repetition; of. this signal; it: will stilt be met and must be recognised as such if incor-- rect operation of. the preceding." circuits is to be avoided, apart altogether from questions as? to the: difficulties of operating procedure. involved by non-repetition of busy flash; It: is: just as dinicult: to= recognise. this; signalin order to prevent:

' misoperation of. the. signaling circuits, asito repeat;

it: backwards; and as this system minimises the latter difliculties it follows. that recognition" and. repetition ofbusy= flash must-be considered;

From Fig.. 8 it; will be seen. that. the total flux;

- roducing currentlobtainedii i :'always R+T irrespective of: the: conditionv of thecontact or batteries, and as thi value is the same as: the totalproduced in ID under? normal loop signaling; then the: same type: of relay can be used.v Hence the normal busy flash signal can be detectedby M, and. it can also be used over. the cable pair whatever the type of impulsive signaling. As rerdsthe ID- this. will. hold in onhalf current when. the contact is. closed, but=will releasewhenthe: contact is. open Itv follows therefore that conditions;- for. clearing. in the intervals of; flashv will be 'satisfactory, if itis arranged tolhold' all the relays during busy flash irrespective. of whether: the: subscriber has. actually cleared; this latter feature: also prevents: unnecessary relay operation; during: busy flash; This explains: the

one winding. of the A. relay, the current? flow through; this: winding; being: such as to hold it operated. irrespective of the condition of the subscribers loop.. ThefllflttlflllX-DI'OdIlCiDg current in this winding: is, of course, the same as that produced inl. M since both. are: due to the: earth current. flowing: between theterminations. The resistances R! are designe'dto hold theA relays. duringithe' transit period: of 'the' MI contact,. and inthetcase: of'the (D/ G set operating! from a subscribers or similar loop-to hold this; relay should the subscriber clear during: busy fiash..

The question. of normal release of.' the circuit. after busy fl'ash: has: been dealt with from the point: of. view of, the guarding, period. on: the private of.' the O/G set. Should however; the. subscriber. clear" during the. interval of busyflash, themp v d'ed the nextz'peri'od. is not received be,- fore the: DD relay-in set-ICR-. has released, the clear is again: normal, since the: release: of. the group. selector: cuts oft. suchtreception Should quired and has no significance.

aeeaeaa (on half current. If itissuccessful in this respect then the sequence is as before, except that now the circuit will have time to clearduring the next complete interval in busy flash. If it is not successful then lack of operation of ID in set OGR is immaterial (because I is releasedhand lack of operation of ID inthe set ICR merely means that the release of DD proceeds normally,

and when it does release, the group selector is released thus cutting ofi busy flash and substituting normal battery if the former is being received from a subsequent selector, if pbusy flash is being received from this g oup selector, again the release cuts it off, and from then on the circults clears down normally.

The manual hold signal and calls to the man ual board via a junction in general will nowbe described. i I i Calls toa manual board in the subscribers exchange present no difficulty of course, since the operator-can be regarded as a localsubscriber, and are routed via the normal equipment. In general, it may be taken as axiomatic that calls to a manual board in a main exchange from a satellite exchange require different backward supervisory signals from ordinary functions, with the sole exception of the normal answer signal. For instance repetition of busy flash is not re- Hence the M relay function can now be used solely for signals such as manual hold and re-ring. Furthermore there is no reason to suspect that such a call will be made over more than one junction, although could be dealt itwill be apparent that the case with. r i

The above does not mean that different electrical signals must now be used, but only that the interpretation must be different. Thus it would be quite legitimate to use the, same signal as for busy flash, were it not for the fact that as it will be continuously applied, the subscriber could not flash the operator. The signal used over the junction in these circumstances, afiords a good demonstration of the inherent flexibility ofthe presentsystem, in that while different from previous signals in its possibilities, it is nevertheless applied in a simple manner and results in no additional complication to the basic circuit arrangements.

The manual hold signal applied to the satellite junction must be capable of allowing the subscriberto flash the operator. Hence use is made of the second alternative given in Fig. 8, in which positive battery replaces the earthen the upper line w1re. Againit will be seen that the flux-producing current in M is i under the first condition but may not operate,

tery F3 via protective resistance R8,.in the case of asatellite junction. At the set OGR the MI contact is now used solely to holdth I relay when the subscriber clears, in order that (a) the D relay may be held operated to send reverse battery to a possible discriminating selector and (b) i to maintain the earth on the private via Cl (C being held by 14 and back contact of All. Of course the polarised A relay in the 1/0 set is held non-operated by the third Winding flux as described before, when the subscriber clears.

The manual hold signal need not be difierent from busy flash, so long as it operates M and permits of subscriber flashing. Double negative can be used from the line tequipment to th I/C set in this case, because two separate line relays are used'and by reference to Fig. 8 for the single counter-E. M; F. case it will be seen that when the contact is open one line current is zero, whilst when it is. closed both line currents are capable of operating-two separate relays. (Note.This condition i always true if the same battery is used for both counter E. M. F.'and line equipment.) Hence if the LC relay in Fig. 9, showing the essentials ofthe line equipment, be connected so that when SS4,SS5 have operated it is in fluenced by the current in the lower line wire,

there the operators cord circuit lamp may be flashed. SS4 and SS5 provide the normal answer signal from the operator when S is operated by the insertion of the answering plug; this signal being necessary for repetition at the O/G set in.

'case a discriminating selector should obtain at the satellite exchange. The calling signal on the line equipment is of course, the operation of L and LC with consequent operation of LL via LCI, and of thelarnp relay LR via SSI, LLI and LI ringing tone being transmitted back to the subscriber via LL4 and SS1. When the subscriber "clears, LC releases, releasing LL, and BH operates via SS3 and LL3; contact BHI then applying negative battery in place of earth for operating theM relays and holding the satellite equipment.

One of the reasons why double negative has been chosen asthe hold signal from the line equipment, is thatithe inclusion of therectifiers MRH and MRIZ in set ICE precludes the use of any backward signalgcontaining positive battery,

and will do both under the remaining three cond1t1ons. The interesting feature is the functioning of the A relay, and it will be seenthat when the contact is open the flux-producing current nected was to changeover to the positive bate owing to the current in the resistance RI tending to be of wrong sense in these circumstances. However, it would have been just as easy to use ,a double earth signal for this purpose, so that this limitation is not very real. Ofcourse under manual hold conditions th control of the group selector must be in the hands of the operator'for clearing, so thatthe incoming private wire of the group. selector must be brought up to the line equipment. The group selector may be held by the earth from SS6 (Fig. 9) under the control of the operator.

With the circuits described,metering is only possible on manual board calls because it is desired to send back to the satellite exchange the reversed battery answer signal so thata standard discriminating selector may operate satisfactorily. But in such circumstances metering is made impossible normally, as is well known, and therefore the same principles can be used in satellite exchanges not having discriminating selectors.

.70 fromthe lines is zero, and when closed is opera- For instance, a simple uni-selector circuit may be arranged to disconnect the meter pulse circuit when the dialled pulses correspond to an operator call.

Other possibilities are just as easily applied, and thereby enhance the value of this system;

no detailed exposition will be attempted since this will be readily apparent to those skilled in the art. I

- One alternative has already been mentioned, namely doubleearth from the A relay end. This signal gives line currents exactly the reverse of double negative, and hence could be used in addition to the latter for the" purpose of providing a different effect. For instance if an additional M relay were used and both were of polarised type, then" one would be operated by double negative and the other by double earth, if appropriately connected. In this way, it is possible to cater for a re-ring signal, without substantial complication to the circuit.

Another signal is double positive from the A relay end, which adds a figure of 100 to the previously quoted relay and line currents, as compared with double negative. For instance, the upper line current in Fig. 8 (for the double negative and taking the first example), becomes R+r I in the reverse direction whilst the lower line current becomes uses in certain instances.

Having introduced a battery on the other side the use of normal loop supervisory backward sigi- 'nals, by the use of a source ofccounter-E; M. F. in place of the conventional disconnection during the break periods of dialled impulses.

- 2. By making the value of the counter-EM. F. twice the nominal battery-voltage at the distant end, a practical form of doublecurr'ent working on impulsing can be obtained :with all the attendant advantages.

3. By compensating the local line supervisory relays, no line switching between the first and of the normal transmission bridge, it is an obvious step to consider whether the junction circuit between the terminating bridges can be made to "work on both way principles. This depends to some extent on the additional complexities of the line relay circuits and the local relay circuits. However, it has been demonstrated that the latter aresubstantially the same already with the exception of the private guarding arrangements, and the inclusion of additional line relays presents no insuperable difficulties, provided use is only made of those necessary for the particular 'directionof signaling under-consideration.

. The necessary switching of the appropriate bate tiery voltages also follows as a function depend- 'ent on which end is seized first, in fact the main difiiculty will be that normal to such circuits, namely prevention of double seizure. In general however, it may be possible merely to add "the necessary additional relays to the present incoming :relay set in order that it can perform the rsameyfunctions 'as the present outgoing set and to switch the transmission bridges over so as to :be facing the right way, and thus save the additional group selector :at -the a'pprc'pr'iate end.

To sillh' it will be appreciated that the invention is' characterised by the :following points which however are b-y-ho means exhaustive.

' 1. The arrival and decay current waveforms at the distant re'lay are made similar and of minimum transient duration, while =yet *unabling l ine relay equipment. The same switching function could, of c'o'urs'e', connect a succeeding impulses is required, hence making .ior uniformity between such impulses. Such compensation may be complete (i; e., so that the relays act as series line resistances) or partial with a view to possible improvement. of the re ceived current waveforms.

. 4-. The signaling loop. is completely isolated from. those on either side by usingtransformer speech transmission .bridges and appropriately connected rectifiers onthe windings during impulsing, so assubstantially to eliminate both longitudinal and transverse surges respectively. Additional and random impulse distortion due to" dialling on associated paths is thereby. rendered negligible. Speech transmission loss .per

junction should not exceed 0.? db. Additional equipment per junction is one simple. I/C relay set.

5. The current drawn from the batteries in the line is small, and in particular the counter- E. M. F. is disconnected during speech.

6. Standard types of relays can be used throughout, all being directly operated from the batteries.

'7. Backward supervisory signals such-as busy flash, manual hold, and re-ring can be repeated with a minimum of circuit complication. The basic relay requirements are three line relays per outgoing or incoming relay set, with six local relays for the former and four for the latter.

:8. Impulse distortion on each repetition at the relay sets can be made and maintained zero; distortion over the junction link is entirely a :matter of the degree of control applied to variables such as battery voltage and polarised relay adjustments (i. -e. lack sures).

' 9. Impulse distortion with short period pulses is within 0.8 'mS over a range of 20 to -60=mS make and 0 to miles for the junction, and I within 0.5 mS for the exchange jpath up to 80 ohms on a through connection.

10. Improvement in the slope and squareness of the received current waveform is perfectly feasible, as is also a better winding for the polarised relay, wi-th the consequence that the above figures can be improved as regards the junction. 1 j

ill. The irnpulsing arrangements into the exchange apparatus, are on a single current'basis and therefore operate this equipment successfully.

12. The limits of operation are dependent only on the distortion tolerable by the ultimate exchange apparatus. Hence.connectionscanbesetup at will and from any intermediate point by normal methods.

l3. The basic designissuch that .checklof the performance .of any signal ,path .or .link can be made rapidly, but should .be required .seldQm. Owing ;to the substantially complete isolatio of each path on dialling, the building .up .of tandem connections impracticable solely by using simple arithmetic as regards overall distortion.

also facilitates the keeping of records.

of equality in contactpres- 14. The basic local relay set design does not require difllcult inter-related release lag timing on the relays; most of the slow release relays perform a standard functions and may be of standard type.

,I claim:

1. In an impulsing systemcomprising a loop circuit terminating at the far end in a line relay having a source of current connected thereto,

means for making and breaking the loop circuit at the near end to cause intermittent operation electromotive force in the loop circuit whenever the same is opened by said first means to aid in the rapid deenergization of said relay.

2. In a telephone system, a line circuit, a relay at one end of'said circuit, a source of current connected in series with the winding of said relay and said line circuit, contacts at the other end of said circuit for opening and closing said circuit to cause energization and deenergizationof said relay by current flowing over said series circuit from said source, and means for introducing a counter-electromotive force in said circuit in opposition to said first source of current, while said contacts are open to thereby aid in the deenergization of said relay.

3. A signaling system for telephone or likesys- -tems comprising a pair of conductors, a line relay in bridge of said conductors and a source of electromotive force in series with said relay, a pair of contacts in bridge of the opposite end a of said conductors, means for opening and closing said contacts, a second source of electromotive force in bridge of said contacts, said second source having a value at least equal to said first source, said contacts, when closed, shunting said second source and causing current to flow through said relay from said first source, said contacts, when opened, causing said second source to be connected to said conductors in a sense opposing the flow of current from said first source to cause said relay to deenergize.

4. An impulsing system as claimed in claim 1 in which the saidcounter-electromotive force is of higher potential than said source of current.

5. A system such as claimed in claim 2 in which said counter-electromotive force is of twice the potential of said source of current.

- of said relay, and means for inserting a counter- 6. A system such as claimed in claim 2 having other relays in said line circuit and means for reversing the connections of said source of current to operate them, said last relays nonresponsiveto current from said counter-electromotive force.

'7. A system such as claimed in claim 3 having an additional relay in said conductors and means for reversing saidfirst source of electromotive force to operate the same, said additional relay having a rectifier in shunt thereof to prevent its operation while said contacts are being operated.

8. In a signaling system, a loop circuit, a relay in series with said loop, a source of current also in series with said loop and said relay, a pair of contacts for opening and closing said loop circuit, a second source of current, and means for introducing said second source in said loop circuit when said contacts are opened to oppose the flower current iromthe first source, an earth connection for each source, the conductors of said loop circuit being balanced with respect to earth, another relay in said loop circuit and rectifiers in shunt of the windings of said other relay to cause it to operate on reversal of said first source but to prevent its operation while said contacts are being operated.

9. In a telephone system, a two wire line cirmm extending to a remote point, a line relay in series with a source of electromotive force in bridge of said conductors at said point, a pair of contacts at the near endof said circuit and means for-opening and closing said contacts to cause impulses of current fromsaid source to traverse said circuit to cause intermittent operation of said relay, a source of counter-electromotive force of higher potential than said remote source, means for connecting said counter-elem tromotive force to said circuit each time said contacts are opened to aid the rapid decay of current flow from saidfirst source, other relays in said line circuit, and rectifiers in shunt of said other relays to prevent operation thereof by current from said'second source.

BERTRAM MORTON HADFIELD. 

